feat: gate download options on real-time yt-dlp video analysis

Options (quality, subtitles) are now derived from a yt-dlp -J probe of
the submitted URL instead of a static list, so users can't pick
combinations the source video doesn't actually support. Submission is
blocked until the probe succeeds.

- New /api/probe endpoint + src/lib/ytdlp-probe.ts, with its own rate
  limiter (rate-limit.ts refactored into a createRateLimiter factory).
- New options: subtitle language selection, clip start/end trim, MP3
  audio quality.
- Fixed buildYtdlpArgs: format=mp3 never triggered audio extraction
  (-x/--audio-format), and quality=best never applied --merge-output-format,
  so the chosen container had no real effect.
- Cross-platform yt-dlp invocation: the bundled bin/yt-dlp is a Python
  zipapp relying on a shebang, which Windows' spawn() can't run
  directly. buildYtdlpCommand() runs it through `python` on Windows and
  execs it directly on Linux/o2switch, where the shebang works natively.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/lib/__tests__/ytdlp-command.test.ts

@@ -0,0 +1,50 @@
+const mockExistsSync = jest.fn()
+jest.mock('fs', () => ({ existsSync: (...args: unknown[]) => mockExistsSync(...args) }))
+
+import { buildYtdlpCommand } from '../ytdlp'
+
+function setPlatform(platform: NodeJS.Platform) {
+  Object.defineProperty(process, 'platform', { value: platform })
+}
+
+describe('buildYtdlpCommand', () => {
+  const originalPlatform = process.platform
+
+  afterEach(() => {
+    setPlatform(originalPlatform)
+    mockExistsSync.mockReset()
+  })
+
+  it('runs the bundled binary directly on Linux', () => {
+    setPlatform('linux')
+    mockExistsSync.mockReturnValue(true)
+    const { command, args } = buildYtdlpCommand(['-J', 'https://x.test'])
+    expect(command).toContain('yt-dlp')
+    expect(args).toEqual(['-J', 'https://x.test'])
+  })
+
+  it('runs the bundled binary through python on Windows', () => {
+    setPlatform('win32')
+    mockExistsSync.mockReturnValue(true)
+    const { command, args } = buildYtdlpCommand(['-J', 'https://x.test'])
+    expect(command).toBe('python')
+    expect(args[0]).toContain('yt-dlp')
+    expect(args.slice(1)).toEqual(['-J', 'https://x.test'])
+  })
+
+  it('runs a system-wide yt-dlp directly on Windows when no bundled binary exists', () => {
+    setPlatform('win32')
+    mockExistsSync.mockReturnValue(false)
+    const { command, args } = buildYtdlpCommand(['-J', 'https://x.test'])
+    expect(command).toBe('yt-dlp')
+    expect(args).toEqual(['-J', 'https://x.test'])
+  })
+
+  it('runs a system-wide yt-dlp directly on Linux when no bundled binary exists', () => {
+    setPlatform('linux')
+    mockExistsSync.mockReturnValue(false)
+    const { command, args } = buildYtdlpCommand(['-J', 'https://x.test'])
+    expect(command).toBe('yt-dlp')
+    expect(args).toEqual(['-J', 'https://x.test'])
+  })
+})